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Terminal or Vercel runtime logs

BetterAuthError: BETTER_AUTH_SECRET is not set or is too short

What it means
Better Auth needs a random secret string (`BETTER_AUTH_SECRET`) to sign login cookies safely, and it hasn't been generated for this environment yet.
Likely causes
  • BETTER_AUTH_SECRET wasn't generated into .env.local or piped into npx vercel env add yet.
  • Only Production was selected when adding BETTER_AUTH_SECRET in Vercel, leaving Preview without one.
Prevention
Have your agent generate BETTER_AUTH_SECRET unseen for both local development and Vercel as soon as logins are added.

Copy for your agent

Better Auth says `BETTER_AUTH_SECRET` is missing. Generate a fresh random secret directly into `.env.local` and into Vercel via `npx vercel env add BETTER_AUTH_SECRET` without printing it in the chat.

Full help articleAn environment variable is “undefined”

Last reviewed Nov 8, 2026